ITU-T Rec. G.1028 (04/2016) End-to-end quality of service ...

I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n

ITU-T G.1028 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU

(04/2016)

SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS

Multimedia Quality of Service and performance – Generic and user-related aspects

End-to-end quality of service for voice over 4G mobile networks

Recommendation ITU-T G.1028

ITU-T G-SERIES RECOMMENDATIONS

TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS

INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100–G.199

GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS

G.200–G.299

INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES

G.300–G.399

GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES

G.400–G.449

COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450–G.499

TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600–G.699

DIGITAL TERMINAL EQUIPMENTS G.700–G.799

DIGITAL NETWORKS G.800–G.899

DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900–G.999

MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE – GENERIC AND USER-RELATED ASPECTS

G.1000–G.1999

TRANSMISSION MEDIA CHARACTERISTICS G.6000–G.6999

DATA OVER TRANSPORT – GENERIC ASPECTS G.7000–G.7999

PACKET OVER TRANSPORT ASPECTS G.8000–G.8999

ACCESS NETWORKS G.9000–G.9999

For further details, please refer to the list of ITU-T Recommendations.

Rec. ITU-T G.1028 (04/2016) i



Summary

Recommendation ITU-T G.1028 provides guidelines concerning the key aspects impacting end-to-end

performance of managed voice applications over LTE networks and how they can be properly assessed

using current elements of knowledge.

Some typical end-to-end scenarios are described, involving cases with LTE access at both sides of the

communication, or with a different access technology at one side (wireless or wireline access). These

scenarios are based on typical reference connections defined in this Recommendation, composed of

various segments, including: terminal, wireless access, backhaul network, core network.

Considerations regarding the sharing of the budget of some key parameters and the location where

they can be assessed across these segments are provided.

History

Edition Recommendation Approval Study Group Unique ID*

1.0 ITU-T G.1028 2016-04-06 12 11.1002/1000/12748

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ii Rec. ITU-T G.1028 (04/2016)

FOREWORD

The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of

telecommunications, information and communication technologies (ICTs). The ITU Telecommunication

Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical,

operating and tariff questions and issuing Recommendations on them with a view to standardizing

telecommunications on a worldwide basis.

The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes

the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics.

The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.

In some areas of information technology which fall within ITU-T's purview, the necessary standards are

prepared on a collaborative basis with ISO and IEC.

NOTE

In this Recommendation, the expression "Administration" is used for conciseness to indicate both a

telecommunication administration and a recognized operating agency.

Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain

mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the

Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some other

obligatory language such as "must" and the negative equivalents are used to express requirements. The use of

such words does not suggest that compliance with the Recommendation is required of any party.

INTELLECTUAL PROPERTY RIGHTS

ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve

the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or

applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of

the Recommendation development process.

As of the date of approval of this Recommendation, ITU had not received notice of intellectual property,

protected by patents, which may be required to implement this Recommendation. However, implementers are

cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB

patent database at http://www.itu.int/ITU-T/ipr/.

ITU 2016

All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior

written permission of ITU.

http://www.itu.int/ITU-T/ipr/

Rec. ITU-T G.1028 (04/2016) iii

Table of Contents

Page

1 Scope ............................................................................................................................. 1

2 References ..................................................................................................................... 1

3 Definitions .................................................................................................................... 3

3.1 Terms defined elsewhere ................................................................................ 3

3.2 Terms defined in this Recommendation ......................................................... 3

4 Abbreviations and acronyms ........................................................................................ 3

5 Conventions .................................................................................................................. 6

6 Brief introduction on Voice over LTE and assumptions .............................................. 6

7 Hypothetical reference models ..................................................................................... 8

7.1 LTE-LTE communication on the same network ............................................ 10

7.2 LTE-LTE communication between two interconnected networks

(includes roaming) .......................................................................................... 10

7.3 LTE-3G communication ................................................................................. 10

7.4 LTE-PSTN communication ............................................................................ 11

8 Aspects of quality of service for voice over LTE services ........................................... 11

9 Relevant indicators and quality targets ......................................................................... 13

10 Recommendations for QoS monitoring and troubleshooting ....................................... 19

10.1 Recommended QoS monitoring points .......................................................... 19

10.2 Monitoring strategy ........................................................................................ 24

10.3 Available tools in ITU-T standards ................................................................ 27

Annex A – List of degradations encountered by end-users of VoLTE service and their

potential causes ............................................................................................................. 31

A.1 QoS problem linked to call session performance ........................................... 31

A.2 QoS problem linked to perceived speech quality ........................................... 32

Bibliography............................................................................................................................. 34

Rec. ITU-T G.1028 (04/2016) 1



1 Scope

This Recommendation works on the assumption that Voice over LTE (VoLTE) is a so-called

''managed'' voice service, in opposition to over the top (OTT) applications without use of session

initiation protocol/IP multimedia subsystem (SIP/IMS) signalling and with no prioritized traffic.

Video-telephony over LTE (ViLTE) is another service which will be addressed in a specific

Recommendation.

This Recommendation describes the key aspects impacting end-to-end performance of managed voice

applications over LTE networks, for the most common call cases (IMS tromboning is not considered,

nor single radio voice call continuity (SRVCC), nor mobility under wireless local access network),

and how they can be properly assessed using current elements of knowledge.

Relevant quality of service (QoS) mechanisms used to manage the voice service, such as robust

header compression (RoHC), transmission time interval (TTI) bundling, semi-persistent scheduling

(SPS), discontinuous transmission (DTX) and reception (DRX), service domain selection (SDS) or

SIP preconditions, are not considered in this Recommendation as a mandatory part of a VoLTE

service, but their impact on end-to-end perceived quality will be taken into account.

Analysis of the impact of VoLTE on quality of supplementary services (such as data streaming) or

on device features (battery life) is outside the scope of this Recommendation.

2 References

The following ITU-T Recommendations and other references contain provisions which, through

reference in this text, constitute provisions of this Recommendation. At the time of publication, the

editions indicated were valid. All Recommendations and other references are subject to revision;

users of this Recommendation are therefore encouraged to investigate the possibility of applying the

most recent edition of the Recommendations and other references listed below. A list of the currently

valid ITU-T Recommendations is regularly published. The reference to a document within this

Recommendation does not give it, as a stand-alone document, the status of a Recommendation.

[ITU-T E.800] Recommendation ITU-T E.800 (2008), Definitions of terms related to

quality of services.

[ITU-T E.804] Recommendation ITU-T E.804 (2014), Quality of service aspects for

popular services in mobile networks.

[ITU-T G.107] Recommendation ITU-T G.107 (2015), The E-model: a computational

model for use in transmission planning.

[ITU-T G.107.1] Recommendation ITU-T G.107.1 (2015), Wideband E-model

[ITU-T G.109] Recommendation ITU-T G.109 (1999), Definition of categories of speech

transmission quality.

[ITU-T G.114] Recommendation ITU-T G.114 (2003), One-way transmission time

[ITU-T G.711] Recommendation ITU-T G.711 (1988), Pulse code modulation (PCM) of

voice frequencies.

[ITU-T G.1000] Recommendation ITU-T G.1000 (2001), Communications quality of

service: A framework and definitions.

2 Rec. ITU-T G.1028 (04/2016)

[ITU-T P.10] Recommendation ITU-T P.10/G.100 (2006), Vocabulary for performance

and quality of service.

[ITU-T P.563] Recommendation ITU-T P.563 (2004), Single-ended method for objective

speech quality assessment in narrow-band telephony applications.

[ITU-T P.564] Recommendation ITU-T P.564 (2007), Conformance testing for voice over

IP transmission quality assessment models.

[ITU-T P.800.1] Recommendation ITU-T P.800.1 (2016), Mean opinion score (MOS)

terminology.

[ITU-T P.862] Recommendation ITU-T P.862 (2001), Perceptual evaluation of speech

quality (PESQ): An objective method for end-to-end speech quality

assessment of narrow-band telephone networks and speech codecs.

[ITU-T P.863] Recommendation ITU-T P.863 (2014), Perceptual objective listening

quality assessment.

[ITU-T P.863.1] Recommendation ITU-T P.863.1 (2014), Application guide for

Recommendation ITU-T P.863.

[ITU-T Y.1540] Recommendation ITU-T Y.1540 (2016), Internet protocol data

communication service – IP packet transfer and availability performance

parameters.

[ITU-T Y.1541] Recommendation ITU-T Y.1541 (2011), Network performance objectives

for IP-based services.

[3GPP TS 22.105] 3GPP TS 22.105 v12.1.0 (2014), 3rd Generation Partnership Project;

Technical Specification Group Services and System Aspects; Services and

service capabilities (Release 12).

[3GPP TS 23.203] 3GPP TS 23.203 v 13.5.1 (2015), 3rd Generation Partnership Project;

Technical Specification Group Services and System Aspects; Policy and

charging control architecture (Release 13).


Technical Specification Group Core Network and Terminals; IP multimedia

call control protocol based on Session Initiation Protocol (SIP) and Session

Description Protocol (SDP); Stage 3 (Release 13).


Technical Specification Group Services and System Aspects; IP Multimedia

Subsystem (IMS); Multimedia Telephony; Media handling and interaction

(Release 13).


Technical Specification Group Services and System Aspects; Terminal

acoustic characteristics for telephony; Requirements (Release 13).

[ETSI TS 101 563] ETSI TS 101 563 v 1.4.1 (2015), Speech and multimedia Transmission

Quality (STQ); IMS/PES/VoLTE exchange performance requirements.

[ETSI TR 103 219] ETSI TR 103 219 v1.1.1 (2015), Speech and multimedia Transmission

Quality (STQ); Quality of Service aspects of voice communication in an

LTE environment.

[IETF RFC 6076] IETF RFC 6076 (2011), Basic Telephony SIP End-to-End Performance

Metrics.

Rec. ITU-T G.1028 (04/2016) 3

[IETF RFC 3095] IETF RFC 3095 (2001), Robust Header Compression (RoHC): Framework

and four profiles: RTP, UDP, ESP, and uncompressed.

3 Definitions

3.1 Terms defined elsewhere

This Recommendation uses the following terms defined elsewhere:

3.1.1 call set-up time [ITU-T E.800]: The period starting when the address information required

for setting up a call is received by the network (recognized on the calling user's access line) and

finishing when the called party busy tone, or ringing tone or answer signal is received by the calling

party (i.e., recognized on the calling user's access line). Local, national and service calls should be

included, but calls to Other Licensed Operators should not, as a given operator cannot control the

QoS delivered by another network.

3.1.2 speech quality [ITU-T P.10]: Quality of spoken language as perceived when acoustically

displayed. Result of a perception and assessment process, in which the assessing subject establishes

a relationship between the perceived characteristics, i.e., the auditory event, and the desired or

expected characteristics.

3.1.3 service accessibility performance [ITU-T E.800]: The ability of a service to be obtained,

within specified tolerances and other given conditions, when requested by the user.

3.1.4 mean one-way propagation time [ITU-T P.10]: In a connection, the mean of the

propagation times in the two directions of transmission.

3.2 Terms defined in this Recommendation

None.

4 Abbreviations and acronyms

This Recommendation uses the following abbreviations and acronyms:

2G Second Generation of radio access network (also called GSM)

3G Third Generation of radio access network (also called UMTS)

4G Fourth Generation of radio access network (also called LTE)

ACK Acknowledgment

AEC Acoustical Echo Cancellation

AF Application Function

AGC Automatic Gain Control

AGW Access Gateway

AMR Adaptive Multi Rate coding

AMR-WB Wide-Band Adaptive Multi Rate coding

BGCF Border Gateway Control Function

BTS Base Transceiver Station

CDR Call Detail Record

CNG Comfort Noise Generation

CS Circuit Switched

4 Rec. ITU-T G.1028 (04/2016)

CSFB Circuit Switched Fall Back

DL Downlink

DRX Discontinuous Reception

DTX Discontinuous Transmission

EEC Electrical Echo Cancellation

E-NodeB Enhanced Node B

EPC Evolved Packet Core

E-UTRAN Evolved UMTS Terrestrial Radio Access Network

EVS Enhanced Voice Services

GBR Guaranteed Bit Rate

GERAN GSM/Edge Radio Access Network

GGSN Gateway GPRS (General Packet Radio Service) Service Node

GSM Global System for Mobile Communications

GSMA The GSM Association

GW Gateway

HARQ Hybrid Automatic-Repeat-Request

HD voice High Definition voice

HO Hand Over

HSS Home Subscriber Server

I-CSCF Interrogating Call Session Control Function

IMS IP Multimedia Subsystem

IPDV Internet Packet Delay Variation

IPER Internet Packet Error Ratio

IPLR Internet Packet Loss Ratio

IPTD Internet Packet Transfer Delay

IRA Ineffective Registration Attempt

KPI Key Performance Indicator

LTE Long Term Evolution (of radio access networks)

MGCF Media Gateway Controller Function

MGW Media Gateway

MME Mobility Management Entity

MOS Mean Opinion Score

MOS-LQ Mean Opinion Score – Listening Quality

MTAS Multimedia Telephony Application Server

MTSI Multimedia Telephony Service for IMS

NB Narrow Band (voice spectrum)

NER Network Error Rate

Rec. ITU-T G.1028 (04/2016) 5

O-SDS Originating Service Domain Selection

OTT Over-The-Top

PCC Policy and Charging Control

PCEF Policy and Charging Enforcement Function

PCRF Policy and Charging Rule Function

P-CSCF Proxy Call Session Control Function

PDD Post Dialling Delay

PGW Packet Data Network Gateway

PLC Packet Loss Concealment

PLMN Public Land Mobile Network

PSTN Public Switched Telephone Network

QCI Quality Classification Identifier

QoS Quality of Service

RACH Random Access Channel

RoHC Robust Header Compression

RRC Radio Resource Control

RTCP Real-time Transport Control Protocol

RTP Real-time Transport Protocol

S-CSCF Serving Call Session Control Function

SCR Session Completion Ratio

SDP Session Description Protocol

SDS Service Domain Selection

SEER Session Establishment Effectiveness Ratio

SGSN Service GPRS (General Packet Radio Service) Support Node

SGW Serving Gateway

SIP Session Initiation Protocol

SIP-I Session Initiation Protocol with encapsulated ISUP

SPR Subscriber Profile Repository

SPS Semi-Persistent Scheduling

SRD Session Request Delay

SRVCC Single Radio Voice Call Continuity

SWB Super Wide Band (voice spectrum)

T-ADS Terminating Access Domain Selection

TDM Time Domain Multiplication

TrFO Transcoding Free Operation

TrGW Trunking Gateway

T-SDS Terminating Service Domain Selection

6 Rec. ITU-T G.1028 (04/2016)

TTI Transmission Time Interval

UDP User Datagram Protocol

UE User's Equipment

UL Uplink

UMTS Universal Mobile Telecommunication System

UTRAN UMTS Terrestrial Radio Access Network

VAD Voice Activity Detection

ViLTE Video-telephony over LTE

VoWiFi Voice over WiFi

VoLTE Voice over LTE

VQE Voice Quality Enhancement

WB Wide Band (voice spectrum)

WiFi Wireless Fidelity radio access network

5 Conventions

None.

6 Brief introduction on Voice over LTE and assumptions

This Recommendation makes the following assumptions, in line with the IMS profile for voice

defined by the global system for mobile communications association (GSMA) in [b-GSMA IR.92]

and the multimedia telephony service for IMS (MTSI) media handling procedures (voice part only)

defined by 3GPP in [3GPP TS 26.114], as far as Voice over LTE (VoLTE) is concerned:

– Voice calls are initiated and/or received by a terminal connected to a 4G radio access network

(evolved universal terrestrial radio access network, or E-UTRAN).

– Terminals connected to E-UTRAN embed a VoLTE client by default, either directly native

on the chip or as a dedicated application.

– E-UTRAN is connected to an evolved packet core (EPC) network. The connecting point is

an enhanced node B (e-NodeB).

– EPC enables the communication between 4G terminals and IMS core platforms, in order to

set up calls using the session initiation protocol (SIP). There is no other possibility to set up

calls.

– EPC supports quality of service (QoS) classification (between policy and charging

enforcement function (PCEF) and the terminal), as defined in [3GPP TS 22.105] (section 5)

and [3GPP TS 23.20]3 (section 6.1.7 and associated table partially reproduced in Table 1).

– SIP signalling is assigned with the QoS quality classification identifier (QCI) 5.

– Real time voice signal during calls is carried out with the real-time transport protocol/user

datagram protocol (RTP/UDP), and is assigned with the QCI 1.

– The application of QCI 5 for SIP signalling bearer and QCI 1 for real time voice bearer gives

them a high priority over all data bearer, resulting in no degradation of the voice service when

used in parallel with data session on the same device.

– Voice calls are possible seamless with any far end user of circuit switched (CS) voice

connected to public switched telephone network (PSTN) or to 2G or 3G mobile access. This

Rec. ITU-T G.1028 (04/2016) 7

implies that the VoLTE service is assumed being deployed over the entire 4G network. Cases

of VoLTE terminals being under 4G but non-VoLTE coverage are not considered.

– Voice calls between two VoLTE clients can be placed using any bit rate and codec mode of

adaptive multi rate coding (AMR) and wide-band adaptive multi rate coding (AMR-WB), as

indicated in section 3.2.1 of [b-GSMA IR.92]. In the near future, the super wide band (SWB)

codec enhanced voice services (EVS) will also be supported. Entities ending the user plane

(terminal, media gateways (MGWs)) support transcoding free operation (TrFO).

– The feature TrFO on SIP session initiation protocol with encapsulated ISUP (SIP-I) is

activated which means that codec negotiation between VoLTE and the other networks is

performed in an end-to-end way.

Table 1 – Quality classification identifiers in use for voice over LTE

(source: [3GPP TS 23.203])

QCI Resource

type

Priority

level

Packet

delay

budget

Packet

error loss

rate

Example services

1 GBR 2 100 ms

(Note)

10−2 Conversational voice

5 Non-GBR 1 100 ms

(Note)

10−6 IMS signalling

NOTE – A delay of 20 ms for the delay between a PCEF and a radio base station should be subtracted from

a given packet delay budget to derive the packet delay budget that applies to the radio interface. This delay

is the average between the case where the PCEF is located "close" to the radio base station (roughly 10 ms)

and the case where the PCEF is located "far" from the radio base station, e.g., in case of roaming with home

routed traffic (the one-way packet delay between Europe and North America west coast is roughly 50 ms).

The average takes into account that roaming is a less typical scenario. It is expected that subtracting this

average delay of 20 ms from a given packet delay budget will lead to desired end-to-end performance in

most typical cases. Also, note that the packet delay budget defines an upper bound. Actual packet delays

– in particular for guaranteed bit rate (GBR) traffic – should typically be lower than the packet delay budget

specified for a QCI as long as the terminal has sufficient radio channel quality.

These elements are constitutive of a so-called "managed" voice service, in opposition to OTT

applications without use of SIP/IMS signalling and with no prioritized traffic.

Relevant QoS mechanisms used in 4G access network or in EPC and IMS platforms to manage the

voice service are not considered in this Recommendation as a mandatory part of a VoLTE service,

but their impact on end-to-end perceived quality will be taken into account as far as possible. The

most frequent of these features are:

– Robust header compression (RoHC, see [IETF RFC 3095]) is a method to shorten the size of

voice packets (header often represents 60% of the total of a packet, and can be reduced by a

factor of up to 20 thanks to RoHC) and thus limit the network bandwidth occupation.

– Transmission time interval (TTI) bundling is a mechanism to improve coverage at 4G cell

edge or in poor radio conditions, by transmitting each packet four times in a row

(without signalling overhead).

– Semi-persistent scheduling simplifies the allocation and re-allocation of radio resources in a

cell and thus can allow more simultaneous VoLTE calls and a lower consumption of battery

in devices.

– Discontinuous transmission (DTX) is a technique to reduce the bandwidth occupied by a

voice call, by not transmitting data during silent periods.

8 Rec. ITU-T G.1028 (04/2016)

– Discontinuous reception (DRX) puts the terminal into periodic repetition of sleep mode and

wake up mode (to listen to messages from the network), allowing saving battery

consumption.

– SIP preconditions, as specified in section 2.4.1 of [b-GSMA IR.92] and [3GPP TS 24.229],

can be added to session description protocol (SDP) messages exchanged between terminals

and IMS in order to ensure that the dedicated radio bearers are available before setting up the

call. This strengthens the call set-up process but makes it longer by up to several seconds.

– Terminating or originating service domain selection (T-SDS and O-SDS) allows selecting

between the CS or the IMS domain to provide the voice service to a VoLTE terminal under

VoLTE and CS network coverage. The mechanism for the selection of domain to

communicate call termination from the network to the terminal is called terminating access

domain selection (T-ADS).

7 Hypothetical reference models

The global architecture supporting VoLTE services can be seen in Figure 1:

Figure 1 – Overall architecture of VoLTE services

Several blocks are present and form the basic elements of a reference model:

– The terminal

– The E-UTRAN

Rec. ITU-T G.1028 (04/2016) 9

– The EPC network

– The mobile IMS core network

– Other radio access networks (GERAN and UTRAN)

– Mobile core networks for 2G and 3G

– Interconnection with other EPC or fixed networks

The description above is simplified in Figure 2:

Figure 2 – Simplified architecture of VoLTE services

This Recommendation provides an overview of the impact of various issues on perceived quality,

together with an estimation of the quantization of this impact by building block of the hypothetical

reference model, for several end-to-end scenarios falling into the scope of this Recommendation.

A client of a VoLTE service can experience different types of calls:

– Basic call: Either with another VoLTE user connected to the same 4G network

(see clause 7.1) or with a user of another voice network (CS or PSTN, see clauses 7.3 and

7.4).

– Circuit switched fall back (CSFB): A call with another 4G terminal, when one of the two

ends must perform a fall back to a CS connection over 3G or 2G before call set up. From a

user point of view, CSFB is automatic and transparent, no action is required. The

performance of CSFB in terms of call set up is seen as a sub-part of basic call performance.

Once the CSFB is performed, the performance objectives in terms of integrity and call

retainability are similar to the ones of a basic call (see clauses 7.3 and 7.4).

– Interconnection: A call between two VoLTE terminals connected to two different

interconnected networks (see clause 7.2).

– IMS tromboning: When the VoLTE terminal is under CS coverage, signalling and user planes

go through the IMS domain. From the e2e performance perspective, this IMS tromboning

should only impact end-to-end delay and post dialling delay (PDD). This call case is outside

the scope of the present Recommendation.

Due to mobility, a call initiated under a 4G VoLTE coverage may have to hand over (HO) to CS

coverage in order to continue. This process, known as SRVCC, is also outside of the current scope of

this Recommendation and under consideration for a further revision.

A call initiated under a 4G VoLTE coverage may also have to hand over to wireless local access

network coverage. A 4G terminal may also directly start a voice call on an IMS platform under this

radio coverage. This use case, known as voice over WiFi (VoWiFi), is also outside of the current

scope of this Recommendation and under consideration for a further revision.

The most common scenarios, considered in this Recommendation, are detailed below.

10 Rec. ITU-T G.1028 (04/2016)

7.1 LTE-LTE communication on the same network

As shown in Figure 3, two clients of the same VoLTE service are in communication. It is assumed

that they are not located under the coverage of the same cell.

Figure 3 – Hypothetical reference model for an LTE-LTE communication on

the same network

7.2 LTE-LTE communication between two interconnected networks (includes roaming)

Figure 4 shows clients of VoLTE services of two different operators in communication. The IMS

core networks are interconnected, allowing both signalling and user plane continuity.

It is assumed that this interconnection is IP-based (time domain multiplication (TDM) solution is not

considered). Also TrFO on SIP SIP-I feature is assumed activated on both networks.

Figure 4 – Hypothetical reference model for an LTE-LTE communication between two

interconnected networks

Two different VoLTE interconnections cases can be addressed:

– The two networks are directly interconnected which yields the use of the same codec – as for

intra public land mobile network (PLMN) case – and low values of PDD and end-to-end

delay.

– The two networks are interconnected through a CS intermediate network that increases PDD

and end-to-end delay, reduces the codec rate, and might even cause the loss of high definition

voice (HD voice) (if WB AMR codec is not supported by the CS network).

7.3 LTE-3G communication

When a VoLTE client is in communication with a CS mobile client, the mobile IMS is connected to

the 3G mobile core network (see Figure 5), allowing both signalling and user plane continuity.

Rec. ITU-T G.1028 (04/2016) 11

Figure 5 – Hypothetical reference model for an LTE-3G communication

7.4 LTE-PSTN communication

When a VoLTE client is in communication with a user on PSTN, the mobile IMS is connected to the

fixed IMS (see Figure 6), allowing both signalling and user plane continuity.

Figure 6 – Hypothetical reference model for an LTE-PSTN communication

All scenarios including interconnection to another network may include very long international paths.

This is considered as a separate case and does not lead to considerations for the general budgeting of

delays.

8 Aspects of quality of service for voice over LTE services

Figure 7 shows the QoS degradations that can be typically encountered on a VoLTE call. QoS is

understood here as defined in [ITU-T E.800]. The main elements of the network are depicted to show

the signalling and media elements as well as the connections with PSTN or mobile platforms.

12 Rec. ITU-T G.1028 (04/2016)

Figure 7 – Typical degradations of VoLTE communications

From a customer point of view (QoS required and perceived, as defined in [ITU-T G.1000]), these

degradations are divided into the following categories:

– Call session performance

– Problems of registration to the service (IMS/SIP).

– Call set up issues (bad accessibility).

– Failed continuity (or retainability), including impact of mobility (radio hand-overs and

SRVCC events).

– Perceived speech quality during the call (integrity)

– Frequency content. This refers to the speech spectrum of signals presented to end users

(NB, WB or SWB) and its potential distortions.

– Interruptions. Concerns all events resulting in clipping of the speech signal during the

conversation.

– End to end delay (impact on conversation interactivity)

– Presence of unwanted noises, from whatever origin.

Rec. ITU-T G.1028 (04/2016) 13

A more detailed list of typical degradations encountered by end-users of VoLTE service, together

with potential causes, is provided in Annex A.

VoLTE can also have an impact on other services or functionalities of the device. For instance:

– data transfer performance is potentially influenced by the presence of VoLTE usage on the

same cell;

– battery autonomy can become critical if the usage of a given application (here VoLTE) is

consuming too much power.

These aspects are however outside the scope of the present Recommendation.

To report, monitor and troubleshoot these QoS issues experienced by customers, consistent

measurements architecture must be set up with appropriate metrics. This is the purpose of following

sections.

9 Relevant indicators and quality targets

From an end-to-end perspective, as perceived by an end-user located under a VoLTE compliant 4G

network coverage and originating calls, the relevant indicators are given in the Table 2, together with

the potential contributor network key performance indicator(s) (KPI(s)).

Table 2 – End-to-end quality indicators and corresponding network KPIs

End-to-end indicators Definition IP network KPIs

Registration success rate Rate of successful registration

attempts in the VoLTE

service

Equivalent to IMS registration

success ratio as defined in

[ETSI TR 103 219]

Registration success rate

KPI related to IMS and based on

P-CSCF counters.

Equivalent to 1 – ineffective

registration attempt (IRA) ratio, as

defined in [IETF RFC 6076]

Service availability End to end service availability in

terms of capacity to establish calls

from, and to, a VoLTE customer.

Equivalent to 1 – VoLTE

session set-up failure ratio as

defined in [ETSI TR 103 219]

Equivalent to 1 – telephony

service non-accessibility as

defined in [ITU-T E.804]

(clause 7.3.6.1)

Network efficiency ratio

Measures the ability of network, from

the service platform point of view, to

deliver calls to the VoLTE customer

Based on SIP protocol, network error

rate (NER) is equivalent to session

establishment effectiveness ratio

(SEER), as defined in

[IETF RFC 6076]

Post dialling delay Time interval (in seconds)

between the end of dialling by the

caller and the reception back by

him of the appropriate ringing

tone or recorded announcement.

Equivalent to call set-up time, as

defined in [ITU-T E.800]

Equivalent to telephony set-up

time as defined in

[ITU-T E.804]

(clause 7.3.6.2)

SIP session set-up time

Interval between sending INVITE

message (with SDP) and ACK (180

or 200) message by the originating

side.

Equivalent to successful session

request delay (SRD), as defined in

[IETF RFC 6076]

14 Rec. ITU-T G.1028 (04/2016)

Table 2 – End-to-end quality indicators and corresponding network KPIs

End-to-end indicators Definition IP network KPIs

Voice quality (MOS-LQ) Equivalent to speech quality

as defined in [ITU-T P.10].

Models like those defined in

[ITU-T P.862] and

[ITU-T P.863] provide an

objective view on the quality

of the voice signal as it may

be perceived by the customer.

Can be seen on a call basis or

on a sample basis

(see [ITU-T E.804]

clauses 7.3.6.3 and 7.3.6.4)

Network quality index

([ITU-T G.107], [ITU-T P.564])

IP Packet loss ratio (see definition

of Internet packet loss ratio (IPLR)

in [ITU-T Y.1540]): several

possible measurement points.

Mouth-to-ear delay The time it takes for the

speech signal to go from the

mouth of the speaker to the

ear of the listener

IP packet transfer delay

(see definition of Internet packet

transfer delay (IPTD) in

[ITU-T Y.1540])

Round trip time

Corresponds approximately to

twice the end-to-end delay

Can be measured based on RTCP

protocol messages

Call drop rate Service continuity in terms of

capacity to maintain calls to their

normal end.

Equivalent to telephony

cut-off call ratio as defined in

[ITU-T E.804]

(clause 7.3.6.5)

Session completion rate

KPI related to IMS and based on

P-CSCF counters

Equivalent to session completion

ratio (SCR), as defined in

[IETF RFC 6076]

Speech bandwidth (NB, WB or

SWB)

Measurement of the

bandwidth used (normal NB

or WB, or even partial and

unwanted bandwidth

limitation).

Codec statistics

Information related to the selection

of (AMR and AMR WB) codec and

codec modes, as well as switch

between them, accessible on SIP

protocol messages.

Measurement points in VoLTE session set-up and completion procedures have been captured in

Figures 8 and 9:

Rec. ITU-T G.1028 (04/2016) 15

Figure 8 – VoLTE session set-up measurement flow

Figure 9 – VoLTE session completion measurement flow

For most of the indicators in Table 2, a budget can be assigned to the various segments that compose

end-to-end paths as seen in clause 6. Tables 3 to 6 provide indications of target values that can be

reasonably reached on each of these segments for each of the hypothetical reference connections

depicted in clause 7. The total budget is not necessarily the exact sum of all individual budgets.

These targets are examples of realistic values that network operators may reach when using tools

complying with up-to-date standards. For instance, the mean opinion scores (MOS) in Tables 3 to 6

are meant as average values when applying [ITU-T P.863] with the right reference sentence (i.e.,

complying with [ITU-T P.863.1]) and doing a small drive test with state-of-the art devices. For longer

16 Rec. ITU-T G.1028 (04/2016)

drive test, with e.g., hand over (HO) events, then these values may be seen as a maximum. For lab

tests with clean stable environment, then this may be seen as minimum value. It should also be noted

that this could change in the future with new devices and network set-ups.

LTE-LTE communication on the same network

Table 3 – Quality budgets for LTE-LTE communication on the same network

End-to-end

indicators

TOTAL

budget

Terminal E-

UTRAN

EPC Mobile

IMS

Transmission

network

Registration

success rate

99.9% 99.9% 99.9% 100% 99.9% No target

Service

availability

99%

Note 1

No target No target No

target

No target No target

Post dialling

delay (PDD)

LTE-LTE:

3.5 s

(Note 2)

CSFB: 6 s

(Note 3)


target

No target No target

Voice quality

(MOS-

LQxSW)

4

(Note 4)


target

No target No target

Mouth-to-ear

delay

400 ms

(Note 5)

190 ms

(sending +

receiving)

(Note 6)

80 ms on

both sides

50 ms 0 10 ms (may be

bigger for large

countries)

Call drop rate 2 % No target No target No

target

No target No target

NOTE 1 – Call processing performance objective according to [ETSI TS 101 563] is higher than 99.9%.

NOTE 2 – [ETSI TS 101 563] recommends 5.9 s, with 95% of probability below 2.4 s.

NOTE 3 – Only circuit switched fall back on mobile originating side is considered here.

NOTE 4 – Assumes the use of AMR-WB at a 23.85 kbit rate. The use of other codecs and/or bit rates will

result in other values.

NOTE 5 – [ITU-T G.114] specifies a preferred maximum value at 150 ms, impossible to reach currently;

some network operators are able to provide national calls with delays below 250 ms.

NOTE 6 – According to [3GPP TS 26.131].

LTE-LTE communication between two interconnected networks (includes roaming)

Table 4 – Quality budgets for LTE-LTE communication between two interconnected

networks

End-to-end

indicators

TOTAL

budget

Terminal E-

UTRAN

EPC Mobile

IMS

(International)

transmission

network

Registration

success rate

99.9% 99.9% 99.9% 100% 99.9% No target

Service

availability

99% No target No target No target No

target

No target

Rec. ITU-T G.1028 (04/2016) 17

Table 4 – Quality budgets for LTE-LTE communication between two interconnected

networks

End-to-end

indicators

TOTAL

budget

Terminal E-

UTRAN

EPC Mobile

IMS

(International)

transmission

network

Post dialling

delay (PDD)

LTE-LTE: 4 s

CSFB: 6 s

(Note 1)

No target No target No target No

target

No target

Voice quality

(MOS-

LQxSW)

4 (if HD voice

+ TrFO)

(Note 2)

2.8 (otherwise)

No target No target No target No

target

No target

Mouth-to-ear

delay

400 ms

(Note 3)

190 ms

(sending +

receiving)

(Note 4)

80 ms on

both sides

50 ms 0 see typical values

in [b-GSMA

IR.34] Table 7

Call drop rate 2 % No target No target No target No

target

No target

NOTE 1 – Only circuit switched fall back on mobile originating side is considered here.






LTE-3G communication

Table 5 – Quality budgets for LTE-3G communication

End-to-

end

indicators

TOTAL

budget

Terminal E-UTRAN EPC Mobile

IMS

(Inter-

national)

trans-

mission

network

Far end

access

network

Far end

terminal

Registrati

on

success

rate

99.9% 99.9% 99.9% 100% 99.9% No target No

target

No

target

Service

availabilit

y

98% No target No target No

target

No

target

No target No

target

No

target

Post

dialling

delay

(PDD)

4.5 s

CSFB: for

further

study


target

No

target

No target No

target

No

target

18 Rec. ITU-T G.1028 (04/2016)

Table 5 – Quality budgets for LTE-3G communication

End-to-

end

indicators

TOTAL

budget

Terminal E-UTRAN EPC Mobile

IMS

(Inter-

national)

trans-

mission

network

Far end

access

network

Far end

terminal

Voice

quality

(MOS-

LQxSW)

3.8 (if HD

voice +

TrFO)

2.8

(otherwise)

Note 1


target

No

target

No target No

target

No

target

Mouth-to-

ear delay

400 ms

Note 2

190 ms

(sending

+

receiving)

Note 3

80 ms 50 ms 0 see typical

values in

[b-GSMA

IR.34]

Table 7

UTRAN

: 90

0

Call drop

rate

3 % No target No target No

target

No

target

No target No

target

No

target






LTE-PSTN communication

Table 6 – Quality budgets for LTE-PSTN communication

End-to-end

indicators

TOTAL

budget

Terminal E-

UTRAN

EPC Mobile

IMS

(International)

transmission

network

PSTN

Registration

success rate

99.9% 99.9% 99.9% 100% 99.9% No target No target

Service

availability


target

No

target

No target 100 %

Post dialling

delay (PDD)

4 s

CSFB:

for

further

study


target

No

target

No target No target

Voice

quality

(MOS-

LQxSW)

3.1

(Note 1)


target

No

target

No target No

degradati

on

Mouth-to-

ear delay

400 ms

(Note 2)

190 ms

(sending +

receiving)

(Note 3)

80 ms 50 ms 0 see typical values

in [b-GSMA

IR.34] Table 7

10 ms

Rec. ITU-T G.1028 (04/2016) 19

Table 6 – Quality budgets for LTE-PSTN communication

End-to-end

indicators

TOTAL

budget

Terminal E-

UTRAN

EPC Mobile

IMS

(International)

transmission

network

PSTN

Call drop

rate


target

No

target

No target 0 %

NOTE 1 – Assumes the use of AMR at a 12.2 kbit rate. The use of other codecs and/or bit rates will result

in other values.




10 Recommendations for QoS monitoring and troubleshooting

10.1 Recommended QoS monitoring points

The measurement points are critical to locate the issues as economical and operational matters

required to limit the number of probes, robots or other tools.

The objective is to ensure a good representation of the covered territory in statistical terms for

reporting, monitoring and troubleshooting with associated measurements sources or tools

(probes, robots, counters and call detail records – CDRs).

Three kinds of measurements points can be envisaged (see Figure 10 below):

– At end points, where end-users access and experience the service (here A and J).

– At interfaces where the user plan is accessible, in general at demarcation points between

network trunks or transmission technologies: here B, D, E and G.

– At points of presence of serving elements concerning signalling plan: here points C, F, and

H (but also E).

20 Rec. ITU-T G.1028 (04/2016)

Figure 10 – VoLTE QoS measurement points

10.1.1 Measurement points A1, A2 and A3

Measurement at the customer premises is absolutely necessary to answer the important need of

knowledge on how the service quality is felt by end users. It gives a real end-to-end visionas there is

no other place to obtain it.

Then comes of course the question of representation: how many measurement points, what

geographical repartition? The answer depends on the possible solutions at the concerned

measurement points.

Different implementation are possible:

A1 measurement point is intended for measurements from a reporting point of view (manual test

campaigns).

In fact, this point is not mandatory for supervision. But from an operator's point of view, a lot of QoS

evaluations with this methodology are done each year on the entire network for regulatory or

benchmark purposes. So, it is a way to check the QoS and to try to resolve encountered problems.

It enables to test end-to-end metrics on accuracy mostly, but also availability, and continuity. Such

methodology is generally based on:

– A good and representative coverage of country area (rural, towns of different sizes, etc.).

– A selection of representative situation of communications (indoor, street, car, train, etc.).

– A selection of representative devices on the operator network.

– A selection of representative day periods to perform calls.

– A selection of representative calls. Narrowband and wideband calls must be taken into

account.

Rec. ITU-T G.1028 (04/2016) 21

– Enough measurements to have a correct view of QoS of the operator and relevant statistics.

A2 and A3 measurement points are intended for measurements where tools replace the end user

present in A1 and model his/her behaviour and quality judgment. They are recommended for

reporting purposes, on end-to-end metrics on accuracy mostly, but also availability, continuity and

service speed. This is where intrusive robots, drive test tools, radio tool analysers and soft agents are

required. The cost of such solution stipulates that only a sampling strategy can be envisaged. The

selection of measurement locations, devices and time periods follow similar criteria as for A1, but

with less constraints since these are automatic tools.

A2 is inside the terminal. There soft agent can access decoded signal and/or information on how the

communication is being processed (call set-up, call drop, IP jitter buffer behaviour, etc.). Such

solutions are generally very cost-effective, can collect data on a very large panel of users and devices,

but may require a participation or feedback of end-users.

A3 is where intrusive tools are required. This can be either static solutions based on robots (the focus

will then be placed on service speed, accuracy and continuity) or mobile solutions (called drive test

tools, with a larger focus enclosing also radio network performance). As seen before, for cost and

operational reasons, only a sampling strategy can be envisaged with such tools (typically less than

ten measurement points for national network). Moreover if a problem is reported by a measurement

tool, it means that many users are probably impacted equally. As a consequence of the low number

of measurements resulting from this sampling strategy, such tools will be mostly used for reporting

purposes. Drive test tools are also commonly used for troubleshooting, when the enhancement of QoS

is linked with the optimization of network coverage.

It is important to understand that this end-to-end vision enables the detection or the confirmation of

a problem but rarely to know the source of the degradation, unless it comes from the device.

10.1.2 Measurement point B

The access part of the network is an important source of outages (biggest number of network

elements). Therefore, its supervision is also necessary. One of the main difficulties is to take into

account the radio part which can be clearly different according to the selected area.

The B point is localized in eNodeB, which is the first ingress point to the LTE network. This

measurement point assumes monitoring the interfaces on both radio and in the EPC direction. The

strategy assumes differentiation of VoIP traffic from other data traffic based on adequate traffic class

indicator (QCI).

It is intended for measurements of:

– radio performance in cells: It is recommended notably for troubleshooting purposes, on

metrics on network performance. Together with A2, the eNodeB point should enable a

correct view of radio coverage performances of area;

– signalling flows, to establish statistics about calls: It is highly recommended for reporting,

monitoring and troubleshooting purposes on end-to-end metrics on service availability,

continuity, accuracy, utilization and network performance.

Supervision tools from RAN providers can enable a global aggregation of those kinds of information

from all eNodeBs.

The measurement strategy for this point should be rather cheap in implementation (i.e., performance

counters accessible from the device) than exhaustive.

10.1.3 Measurement points C1, C2 and C3

Points C1 to C3 corresponds to the interfaces of EPC (in fact all are localized close to mobility

management entity (MME)) where most important signalling messages can be captured and analysed.

They are recommended for reporting, monitoring and troubleshooting purposes on end-to-end metrics

22 Rec. ITU-T G.1028 (04/2016)

on service availability, continuity, and utilization. An exhaustive access to all data for all session is

recommended, mostly through the usage of probes or network element call detail records (CDRs).

C1 covers signalling between MME and radio access (interface S1), C2 supervises the link to serving

GW (interface S11), whereas with C3 the Sgs interface with MSC server is also addressed.

Please note that the traffic captured on all interfaces covered by C points is tunnelled (GTP v2-C).

These measurement point are important especially for the analysis of:

– EPS bearer statistics.

– Mobility management (HO statistics, routing area update).

– Subscriber management (direct connection to HSS).

10.1.4 Measurement points D1 and D2

Like C1, C2 and C3 these points are located inside the EPC, but covers interfaces where real time

flows (using the RTP protocol) can be captured and analysed.

D1 is positioned at the egress between EPC and radio access (interface S1u of serving GW), whereas

D2 covers the link between serving and PDN GWs (interfaces S5-S8).

The serving GW constitutes the anchor point for intra-LTE mobility, as well as for mobility between

GSM/GPRS, 3G/HSPA and LTE. It supports transport level QoS through marking IP packets with

appropriate DiffServ code points based on the parameters associated with the corresponding bearer.

The PDN GW is the point of interconnect to external IP networks through the SGi interface. It also

has a key role in supporting QoS for end-user IP services.

With such critical measurement points, all information on the quality of transport of communication

is accessible. They are highly recommended for reporting, monitoring and troubleshooting purposes

on end-to-end metrics on service availability, continuity, accuracy, speed and network performance.

An exhaustive access to all data for all session is recommended, mostly through the usage of probes

or network element CDRs.

10.1.5 Measurement point E

Measurement point E is located at the border between EPC and 3G core network. The corresponding

equipment is the MSC server, which controls calls and mobility in 2G and 3G networks (e.g., in case

of SRVCC). The MSC server is also in charge of TFO and TrFO functions. In many cases, MSC

server is collocated with media gateway controller function (MGCF), in charge of creating and

releasing connections at MGWs between IMS and CS domains.

This point is therefore recommended for monitoring and troubleshooting purposes on most metrics

concerning communications where roaming between LTE and 2G or 3G occurs. The concerned

metrics fall in the service availability, continuity, and speed families mostly.

10.1.6 Measurement point F

The policy and charging rule function (PCRF) is the central entity in policy control making policy

and charging control (PCC) decisions. The decisions can be based on input from a number of different

sources, including:

– Operator configuration in the PCRF that defines the policies applied to given services.

– Subscription information for a given user, received from the subscriber profile repository

(SPR).

– Information about the service received from the application function (AF).

– Information from the access network about what access technology is used, etc.

Rec. ITU-T G.1028 (04/2016) 23

The measurement point F is meant for monitoring the Rx interface between PCRF and the IMS

domain proxy call session control function (P-CSCF). It is recommended for reporting, monitoring

and troubleshooting purposes on metrics on service utilisation, availability, continuity, and speed.

10.1.7 Measurement points G1, G2 and G3

This group of measurement points corresponds to media gateways (GWs). GWs are important

locations to have specific information about evaluation of core network status (IP packets losses,

delay and jitter), codec in core, transcoding and use of TrFO. These are points of interest for

monitoring and troubleshooting purposes on metrics such as service accuracy and network

performance.

Proper monitoring of media flows (RTP) not only for interworking of services between EPC and other

networks is supported by various types of GW:

– G1 – IMS MGW – this is a GW used for anchoring media legs between two VoLTE users

but also in case of SRVCC (HO from VoLTE to mobile CS) or VoLTE – legacy mobile CS

interworking. This gateway should also provide functionality for interworking between

VoLTE and fixed VoIP users (with AMR/AMR-WB – ITU-T G.711 transcoding).

– G2 – PSTN MGW – this is a place where monitoring, reporting and troubleshooting can be

applied (for network performance), for outgoing IP (from IP to PSTN) flows. This is also

(and this is its most interest) the place where the performance of cancellation of echoes

coming from PSTN is performed.

– G3 – Trunking gateway (TrGW) in case of interconnection with another IP domain operator.

10.1.8 Measurement Points H1, H2, H3 and H4

This is the group of measurement points inside IMS platforms. Monitoring information which can be

possessed here are related to the signalling traffic. Probes which can be deployed at the platform side

should be complementary to the information received from CDRs and counters. Monitoring

information which should be gathered at the corresponding measurement points are related to service

availability, continuity and utilization. According to the service (voicemail, conferencing, etc.),

accuracy and network performance supervisions can also be interesting to track audio problems.

Instead of having one single point, it is recommended to separate the capture of metrics at the level

of different entities:

– H1 – is located at the interface between EPC (PDN GW) and IMS (P-CSCF). This point

covers passive monitoring of Gm interface, which is the first interface where none tunnelled

SIP signalization coming from all users in the network is centralized;

– H2 – is a global monitoring point localized on IMS platform (comprising at least elements

such as I- and S-CSCF, AS and SCC-AS), ideal to make reporting on service utilization as

the analysis will be highly statistically reliable. Monitoring data gathered in this point are

focused on performance counters and billing data. As the most suitable platform element for

gathering statistics may be metric dependent, detailed information on collection interface will

be specified in metrics definitions;

– H3 – measurement point (on call server/MGCF) can be used for an exhaustive view on the

signalling protocol concerning all calls outgoing to PSTN network, since the call server is

involved in all call negotiations. Service availability metrics (and other with less priority

concerning continuity and network performance) are measurable at this point;

– H4 – controls IBCF, where the interconnection between two operator domains is performed

(including charging data records). SIP signalling information for interconnection can be

monitored here.

24 Rec. ITU-T G.1028 (04/2016)

10.1.9 Measurement point J

The J measurement point is the circuit-switched network counterpart of A point. The same metrics

can be measured for similar purposes (except radio network performance), and with the same

sampling strategy.

"Circuit switch", in this case means all possible interfaces: PSTN, 2G/3G PLMN as well as fixed

VoIP access behind an FXS port of a DSL router.

10.2 Monitoring strategy

This clause describes the location of the metrics that have to be measured according to their usage.

In particular, it aims at defining:

– what needs to be measured, how it will be measured (with what source of information, at

what frequency) and why these measurements are needed;

– what kind of measurement tools and where (locations description and number) these

measurements are needed.

10.2.1 Reporting

The reporting will provide a general view on the service QoS, potentially in a benchmark perspective

(between countries, or against domestic competitors), and on its evolution in time (longitudinal view).

It can also allow identification of categories of customers that have a degraded QoS.

Rec. ITU-T G.1028 (04/2016) 25

Figure 11 – VoLTE QoS measurement points – reporting

All measurement points can actually provide data for reporting. Even not fully representative data,

like those gathered from intrusive measurements, can be valuable for this purpose.

The following dashboards can be built based on such measurements:

– General view of the service utilisation (number of customers, number of calls, call durations,

churn rate).

– Performance of service platforms and network equipment (service availability and

continuity).

– QoS counters (availability, PDD, mean opinion score (MOS), call continuity).

26 Rec. ITU-T G.1028 (04/2016)

10.2.2 Monitoring

All measurement points can provide data for monitoring, as long as they can provide a view on either

end-to-end QoS or the contribution of a network section to this QoS.

The hot mode monitoring (alarm raising) will provide real time information related to degradation of

the service QoS impacting a great number of customers.

Figure 12 – VoLTE QoS measurement points – monitoring

10.2.3 Troubleshooting

The troubleshooting will provide additional information by:

– In depth analysis of customer or access below the threshold, by trying to reproduce the causes

for the encountered problems and analysing data collected during the problem by different

sources (CDRs, probes).

– Remote testing/configuration checks, including trace capture and analysis at major network

nodes.

Rec. ITU-T G.1028 (04/2016) 27

– On site investigations, including trace capture at the customer premise or in the access

network.

Figure 13 – VoLTE QoS measurement points – troubleshooting

10.3 Available tools in ITU-T standards

Though based on new network technologies, VoLTE remains a telephony offer subject to all

requirements expressed by end users concerning its usage, and in particular when it comes to QoS.

Therefore, provisions already contained in existing ITU-T Recommendations concerning the

procurement and evaluation of perceived quality of voice services apply in most cases to VoLTE as

well.

10.3.1 Definition of key performance indicators

As can be seen in previous sections (in particular in Table 2), the metrics representative of the quality

of service as perceived by end users are not specific to VoLTE. Most of them are already defined in

28 Rec. ITU-T G.1028 (04/2016)

[ITU-T P.10], and these definitions apply for this context. Following are the details of these

references:

– Service availability: see service accessibility performance (clause 3.1.1.2.2 in

[ITU-T E.800]).

– Post dialling delay: see call set-up time (clause 3.1.1.2.1 in [ITU-T E.800]).

– Voice quality (MOS-LQ, see ''Speech Quality'' (S-28) in [ITU-T P.10] or the various

definitions provided in [ITU-T P.800.1].

– Mouth-to-ear delay: not defined as such in ITU-T Recommendations. See however mean

one-way propagation time (M-1) in [ITU-T P.10], as well as explanation in [ITU-T G.114].

– Call drop ratio: see telephony cut off call ratio in [ITU-T E.804].

– Speech bandwidth (NB, WB or SWB): amendment 4 of [ITU-T P.10] gives very detailed

definition of various audio bandwidth used in telephony.

Registration success rate for VoLTE is not defined in Recommendations. [ITU-T E.804]

(clause 7.2.2.1) defines a network selection and registration failure ratio, but this disregards the

specificity of IMS registration.

As far as the main QoS dimensions are concerned, the four viewpoints of QoS (see Figure 14) defined

in [ITU-T G.1000] are also useful in the context of VoLTE.

Figure 14 – The four viewpoints of QoS (source: [ITU-T G.1000])

As far as it concerns metrics representative of the underlying network layers, [ITU-T Y.1540]

provides information on IP-related metrics, while no ITU-T standards address radio metrics.

10.3.2 Tools and models for the measurement and prediction of voice quality

There are two approaches for the assessment of end-to-end voice quality:

– Parametric tools take advantage of the good correlation between technical information of a

connection and the corresponding end-to-end quality as perceived by end-users, to produce

a relatively accurate estimate at a cheap implementation cost. Such a tool can be envisaged

at edge points, close to the end-user, for a better prediction of individual quality, or inside the

network, for a good knowledge of the general impact of network performance on end-to-end

quality. [ITU-T P.564] describes a general class of parametric voice quality prediction

models that provide highly scalable voice quality estimation using information in the

IP/UDP/RTP header of packets. In addition, [ITU-T P.564] provides performance criteria for

models of this type that operate on narrowband speech.

Rec. ITU-T G.1028 (04/2016) 29

– Another type of parametric tool is the E-model described in [ITU-T G.107], which is a widely

used transmission planning tool. Most factors present in the model (now adapted for IP

transmission and WB telephony, see [ITU-T G.107.1]) apply for VoLTE.

– Signal-based models are much more complicated than parametric tools since they require a

capture and an analysis of the speech signal. This is why they are mostly used in a point-to-

point perspective, in order to measure very accurately the end-to-end quality of a voice

service at a given time and at a given location (and most of the time for a given service with

a given end device). ITU-T developed several such tools: [ITU-T P.862] and [ITU-T P.863]

for full-reference models, and [ITU-T P.563] for a single-ended implementation (restricted

to narrowband telephony).

10.3.3 Applicable acceptability thresholds and targets

In general, ITU-T do not specify acceptability targets for end-user's metrics. However, there is a

notorious exception for end-to-end delay, where [ITU-T G.114] specifies (in clause 4) a first threshold

at 150 ms below which most users do not notice the delay, and a second one at 400 ms above which

the quality is considered as unacceptable.

R factor values computed with the E-model from [ITU-T G.107] (and translated into MOS-CQE

scores, as shown on Figure 15) can also be compared with acceptability thresholds. Recommendation

[ITU-T G.109] defines such categories (for NB telephony only) reproduced in Table 7:

Table 7 – Definition of categories of speech transmission quality (source: [ITU-T G.109])

R-value range Speech transmission

quality category User satisfaction

90 R < 100 Best Very satisfied

80 R < 90 High Satisfied

70 R < 80 Medium Some users dissatisfied

60 R < 70 Low Many users dissatisfied

50 R < 60 Poor Nearly all users dissatisfied

Figure 15 – MOS-CQE as function of rating factor (source: [ITU-T G.107])

Recommendation [ITU-T Y.1541] also provides performance objectives for various QoS classes (see

Table 8) based on IP-network metrics defined and specified in [ITU-T Y.1540]. VoLTE falls into

class 0 or 1:

30 Rec. ITU-T G.1028 (04/2016)

Table 8 – IP network QoS class definitions and network performance

objectives (source: [ITU-T Y.1541])

Network performance

parameter Nature of network performance objective

QoS classes

Class 0 Class 1

IPTD Upper bound on the mean IPTD 100 ms 400 ms

IPDV Upper bound on the 1 10–3 quantile of IPTD

minus the minimum IPTD

50 ms 50 ms

IPLR Upper bound on the packet loss probability 1 × 10–3 1 × 10–3

IPER Upper bound 1 × 10–4

Rec. ITU-T G.1028 (04/2016) 31

Annex A

List of degradations encountered by end-users of VoLTE service and their

potential causes

(This annex forms an integral part of this Recommendation.)

A.1 QoS problem linked to call session performance

Table A.1 – degradations related to call session performance and their potential causes

Kind of degradation Possible reasons: Location

Identification Failure – Problem with MME, HSS or PCRF EPC

Unavailability of

basic call

– Error in scheduling

– Radio resource control (RRC) connection set-up failure

(reception of RRC connection reject, or expiry of timer

T300, no RRC connection set-up complete sent after

reception of RRC connection set-up).

eUTRAN

– Not available due to load (serving gateway (SGW) or packet

data network gateway (PGW))

– Failed negotiation (no allocation of QCI, no codec match,

SIP preconditions unmet, etc.)

– Reception of several SIP error codes (e.g., 401 =

Unauthorized, 405 = Method Not Allowed, etc.)

– Reception of SIP CANCEL from IMS

– TD internal timer expired, causing a

''SessionSetupFailureTimeout''

EPC

High post dialling

delay

– Load

– Interworking between systems

– Use of SIP preconditions

– CS fall back or IMS tromboning at call set-up

All

Link failure – Bad negotiation between two equipments of the network

during call establishment (bad codec management) eUTRAN/ EPC

White call – Terminal is not able to code or decode speech while the

signalling is OK for the communication Terminal

Call drop

– Terminal bug, bad covered area, handover/SRVCC failures

due to cells neighbourhood problem, etc.

– RRC connection drop (at reception of RRC connection re-

establishment reject, or expiry of timer T301 or in case RRC

connection release is received before new RRC connection

set-up attempt)

Terminal/

eUTRAN

– Link failure: System failure, bad re-negotiation between two

equipments of the network during call.

– Reception of SIP status code 500 (Server Internal Error)

– No RTP packet received during a period longer than

''SessionDropTimeout'' TD internal timer

– No SIP 200 OK on BYE is received within the time measured

by ''SessionHangupTimeout'' TD internal timer

EPC

32 Rec. ITU-T G.1028 (04/2016)

A.2 QoS problem linked to perceived speech quality

Table A.2 – Degradations related to perceived speech quality and their potential causes

Kind of

degradation

Possible reasons: Location

Noise

– Disturbing comfort noise generation (CNG) due to bad noise

reduction.

Terminal

– Noise due to bad electronic implementation on terminal (e.g.,

analogue /digital conversion).

– Disturbing residual noise due to bad noise reduction.

– Background noise (street, car, etc.).

– Additional noise due to eUTRAN configuration problem.

Echo

– Bad performance of acoustic echo cancellation (AEC)/ No AEC. As

reminder: Acoustic echo is the coupling between the loudspeaker

and the microphone of the phone terminal.

– Bad performance of electric echo cancellation (EEC)/No EEC .

Reminder: Electrical echo is due to digital to analogue

transformation for a call between mobile terminal and PSTN (No

electrical echo for mobile to mobile call).

Networks

Low/high speech

level – Bad performance of automatic gain control (AGC)/No AGC. Terminal

Encoding /

decoding issues

– Narrowband instead of wideband speech quality:

• Remote terminal not WB

• HO towards 2G

• Call with PSTN, 2G, mobile platforms, etc. where wideband

is not deployed.

• Interworking with CS 3G not WB

Terminal/

eUTRAN

– Lower WB-AMR bitrate/AMR (Loaded cell, autonomous mode,

etc.) leading to distortion on speech signal.

– Many transcodings (for example with call to voicemail) leading to

distortion on speech signal

– Rebuffering and time scaling causing distortion

Terminal/

eUTRAN

Terminal Acoustic

– Although WB-AMR codec is supported, the acoustical performance

of the terminal (on receiving and/or sending side) is not wideband

compliant.

– Not well-balanced acoustic terminal can lead to a sound which seems

too aggressive, too muffled, etc.

– Distortion due to transducers.

Terminal

Chopped

Conversation

– Bad VAD/DTX/DRX implementation.

– Problem with voice quality enhancement (VQE) algorithm. Terminal

– IP packet loss or jitter in network (congestion, QoS prioritization,

UL/DL scheduling delays, radio retransmissions, handover).

– Bad handling of IP packet loss and inter-arrival jitter by jitter buffers

or packet loss concealment (PLC) inside terminals

All

DTMF not

recognized – Problem with in-band or out-band processing All

Rec. ITU-T G.1028 (04/2016) 33

Table A.2 – Degradations related to perceived speech quality and their potential causes

Kind of

degradation

Possible reasons: Location

E2E delay

– Network load.– Media handling (packet construction, jitter buffer

management)

– Speech processing in terminals

– Random access channel (RACH) upon receiving handover

command

– RACH/contention procedure

– Additional RACH attempts

– Dynamic scheduling, link adaptation

– Radio link failure/re-establishment during handover

(possibly different cell)

All

RTP/IP packet

loss

– Network congestion (several causes : traffic load, distance from cell

centre causing activation of TTI bundling, for instance)

– Jitter buffers not adapted to actual jitter amount or packet size

(can depend on use of RoHC or not)

EPC/

Terminal

RTP/IP

Desequencing – New route after a problem such as congestion EPC

Network Delay

Variation (Jitter)

– Network congestion

– Jitter buffers not adapted

EPC/

Terminal

Radio

degradations

– Limit of the cell coverage

– Interference

– Area not well covered (obstacle, etc.)

– Bad radio optimization

– Radio loss profile

– Bad radio scheduling

– No or bad use of hybrid automatic-repeat-request (HARQ)

mechanisms

– Etc.

eUTRAN

Handover latency – Latency due to new route after HO or SRVCC EPC/CS

network

34 Rec. ITU-T G.1028 (04/2016)

Bibliography

[b-GSMA IR.34] GSMA IR.34 v 9.1 (2013), Guidelines for IPX Provider networks.

[b-GSMA IR.92] GSMA IR.92 v 7.0 (2013), IMS Profile for Voice and SMS.

Printed in Switzerland Geneva, 2016

SERIES OF ITU-T RECOMMENDATIONS

Series A Organization of the work of ITU-T

Series D General tariff principles

Series E Overall network operation, telephone service, service operation and human factors

Series F Non-telephone telecommunication services

Series G Transmission systems and media, digital systems and networks

Series H Audiovisual and multimedia systems

Series I Integrated services digital network

Series J Cable networks and transmission of television, sound programme and other multimedia

signals

Series K Protection against interference

Series L Environment and ICTs, climate change, e-waste, energy efficiency; construction, installation

and protection of cables and other elements of outside plant

Series M Telecommunication management, including TMN and network maintenance

Series N Maintenance: international sound programme and television transmission circuits

Series O Specifications of measuring equipment

Series P Terminals and subjective and objective assessment methods

Series Q Switching and signalling

Series R Telegraph transmission

Series S Telegraph services terminal equipment

Series T Terminals for telematic services

Series U Telegraph switching

Series V Data communication over the telephone network

Series X Data networks, open system communications and security

Series Y Global information infrastructure, Internet protocol aspects and next-generation networks,

Internet of Things and smart cities

Series Z Languages and general software aspects for telecommunication systems

ITU-T Rec. G.1028 (04/2016) End-to-end quality of service ...

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